The splitting of an incoming modulated light beam into two outgoing beams is conventionally achieved by joining a primary light guide, carrying the incoming beam, to a pair of secondary light guides designed to convey respective portions of the luminous energy to their destinations. The extremities of the secondary light guides confronting the primary light guide must be suitably reduced for proper splicing. The resulting structure is generally Y-shaped, with the arms of the Y diverging at a small angle from each other on account of the transmission characteristics of a light guide which enable propagation of light rays only within a narrow field bounded by limiting ray paths including a predetermined critical angle with the guide axis. At such a junction, therefore, splitting of the beam energy between two branches is possible only in the direction from the primary to the secondary guides since any beam incoming through one of the arms of the Y could not be divided between the other arm and the stem thereof.
The use of semitransparent reflectors for beam-splitting purposes is also known but generally entails significant losses of luminous energy, especially when such a beam splitter is to be utilized for combining two contributing beams into a composite beam.